Self-priming pumps are known in the prior art and have been used for many years successfully in the processing industry. In this context, the processing industry means in particular the beverage industry, food industry, pharmaceuticals and biochemistry.
Such pumps are for example designed as self-priming rotary pumps. A first chamber and second chamber in each of which an impeller is arranged can be provided between the inlet and the outlet of such a rotary pump. Each impeller is part of a pump stage, wherein the pump stage closer to the inlet generates the self-priming feature.
A first self-priming rotary pump of this kind is proposed in EP 1 191 228 A2. Another self-priming pump is described in DE 10 2007 032 228 A1.
In addition to the rotary pump stage, these two rotary pumps possess a liquid ring pump stage that receives the fluid to be pumped directly from the inlet of the rotary pump. With the assistance of the liquid ring pump stage, an underpres sure can be generated that draws the liquid from the line connected to the inlet.
A return line connects the overpressure region of the rotary pump stage to the inlet of the liquid ring pump stage. This ensures a liquid reservoir, which is needed when starting the rotary pump to generate the liquid ring.
The impeller of the rotary pump stage is connected to the impeller of the liquid ring pump stage via a shaft section that penetrates an opening in a housing wall. In both cases, the shaft section is designed cylindrically up to its shaft end facing the impeller of the rotary pump.
An important parameter of such self-priming rotary pumps is the NPSH value. NPSH stands for “net positive suction head” and is frequently equated with the term “maintained pressure level”. This parameter indicates the overpressure of the fluid to be pumped at the inlet of the pump that must predominate about the vapor pressure of this fluid in order to prevent cavitation in the pump interior. This increased pressure must be generated in the processing system. A pump is therefore sought that has the lowest possible NPSH value.